Lower international carbon prices could wipe between $8 billion and $16.5 billion in revenue from the carbon price scheme up until 2018, modelling done for the nation’s biggest electricity generator and the NSW government says.

The leaking of the modelling to the federal opposition comes amid new pressure on the Gillard government over the high three-year fixed price starting at $23 on July 1, given global prices are now about $10.

If the international price remains low – about $7.40 in 2015-16 rising to $12.52 in 2020 – up to $16.5 billion could be wiped from budget revenue expected under a $29 carbon price.

This figure is based upon no assistance for power generators or emissions intensive trade exposed industries and companies in those sectors having to buy all their permits.

If industry assistance continued after 2015-16 revenue would be up to $8 billion less until 2018-19.

But in Question Time yesterday Treasurer Wayne Swan stood firm and threw doubt on the modelling as the opposition and shadow treasurer claimed Labor was facing a massive budget black hole.

“The fact is that the NSW government has been publishing all manner of modelling in recent times which is, as I just described, bodgie modelling,” Mr Swan said.

“We have full faith, complete faith, in our modelling.”

In modelling done by the Federal Treasury last year, it was projected the carbon price would rise to $29 in 2015-16 when trading starts, off the back off a strong international price.

At that time, companies would be able to buy up to 50 per cent of the permits they required internationally but would face a floor price of $15 for the first three years of trading.

However, since the modelling was released, the price of carbon in the European Union trading schemeand the United Nations clean development mechanism has plunged as a result of the economic slowdown arising from the European debt crisis.

The European Union is now considering reducing the number of available permits to increase the price of carbon.

Federal Treasury has not yet provided carbon revenue estimates after 2014-15 and has previously refused to speculate on the budgetary impact of a lower than expected carbon price, given the potential for a reduction in assistance to industry.

But it confirmed earlier this year, that it was preparing carbon revenue estimates for this year’s budget for 2015-16, the first year of emissions trading.

The release of the NSW government modelling came after the Minerals Council of Australia released a study by the Centre for International Economics finding that over the first six years of the scheme, the impact of the carbon scheme on the economy would be $30 billion greater than if there was not a fixed price and then price floor.

The CIE research –using the G-Cubed global model used by Treasury – assumes over this period a flat international price of $10 in real terms during the first six years of the carbon scheme and stagnant global growth.

But CIE executive director David Pierce said investors were interested in the long-term carbon price, given that power assets had a 30- to 40-year life span.

“[Also] if you look to a high price and then its going to drop then that is an interesting kind of certainty,” Mr Pierce said. “I don’t fully understand that argument or how to measure the gains for that.”

Finance Minister Penny Wong said the government stood behind the federal Treasury modelling.

Last year, estimates done for the Business Council of Australia indicated a low permit price would wipe $3 billion annually from 2015-16 to 2019-20.

BCA chief executive Jennifer Westacott said given the possible fall in revenue it was now essential the industry assistance was not put at risk given the household compensation package has been “hardwired’.

“This will put even more pressure on a what may well be difficult future budgets,” Ms Westacott said.

“Governments need to put out all stops to ensure the Australian economy can grow and Australia’s businesses are competitive and ensure public expenditure is reigned in.”